1. Field of the Invention
The present invention relates to welding equipment and more particularly to a doughnut-type transformer for resistance butt welding. Such transformers are utilized in resistance butt welding machines, specifically, for joining pipes.
This invention may prove most advantageous in resistance butt welding machines which are to be arranged within the pipes to be welded, i.e. in the case when a transformer of a specified rating is required to be as small in terms of its weight and dimensions as possible.
2. Description of the Prior Art
Known in the art is a doughnut-type transformer for resistance butt welding (USSR Inventor's Certificate No. 178429, Int. Cl. B 21 k 09/00, 1966), wherein the secondary winding comprises two turns in series, shaped as coaxial cylinders and placed around the core and the primary winding, and thus providing a sealed double-walled frame with a cooling liquid circulating between the walls thereof.
Though offering an improved cooling system this transformer is not deprived of shortcomings which present a problem in application. The problem resides in the fact that the secondary winding in the form of two cylinders has an increased effective resistance as compared with the transformers of a single-turn design. An increase in the effective resistence of the secondary winding of the transformer limits its applicability, i.e. the range of pipe sections for welding gets decreased.
Also known in the art is a transformer for resistance butt welding (USSR Inventor's Certificate No. 93847, Int. Cl. B 23 k 11/24, 1951), comprising an annular core surrounded by transformer sections each having a primary winding and a secondary winding provided with contact shoes. The core is preferably a ring shaped as a regular polygon each side of which is surrounded by the primary winding which is further surrounded by the secondary winding. The prior art transformer may be used in both types of resistance butt welding machines, namely those for operation within the pipes and outside the pipes, as may be required by the terminal design.
The secondary winding of the above transformer may be cooled either naturally or forceably as the case may be, the manner of cooling being effected by means and ways widely known in the art of electrical engineering.
Since the secondary winding of the transformer consists of only one turn, its effective resistance is within the allowable range but in commercial use it presents a problem which is as follows.
Inasmuch as the windings and the core are concentric and the secondary winding is placed around the primary winding, the latter is always shorter than the former.
The effective resistance is therefore increased in the secondary circuit of a welding machine which overheats the transformer.
Furthermore, in the concentric arrangement of the windings the secondary turn is far from the core, which results in power losses.
The fact that the transformer sections are arranged on an annular core having the form of a circle or polygon is the cause of voids not filled with an active material, such as copper or iron. It is only natural that the copper space factor of the transformer is very low.
The problem also resides in that the prior art transformer for use with the welding machines which are to be operated within the pipes to be welded requires a forced cooling system and attempts to provide the same have failed. Should passages (pipe lines) for a cooling medium be provided, while the windings left invariable, then the outer dimensions of the transformer will be greatly increased though limited by the inner diameter of the pipes to be welded. On the other hand, any cavities in a winding to provide cooling and to retain dimensions of the windings will decrease the quantity of the active material (copper) and increase electric resistance of the winding.
This transformer is a sophisticated design to assemble. Difficulties are met with in mounting concentric windings having a large length when assembled, on the annular core.